General: Voltmeter Impedance

Why is high input impedance desirable for a voltmeter? [G4B05]

A. It improves the frequency response
B. It decreases battery consumption in the meter
C. It improves the resolution of the readings
D. It decreases the loading on circuits being measured

Remember that impedance is the property of a circuit that prevents current from moving through a circuit. When measuring a circuit, we do not want to alter the operation of the circuit. In practice this is impossible, there will always be a certain amount of change when we measure something.

However, by using a voltmeter with a high input impedance, we prevent the current in the circuit from splitting off into the meter. “Path of least resistance” and all that, or in our case, impedance. If we were to attach a low-impedance meter to the circuit, it would “load” or pull power away from the circuit.

The answer then, is D. It decreases the loading on circuits being measured.